1. Field of the Invention
The present invention relates to a Raman amplifier that is used in an optical communication system and that amplifies signal light in an optical fiber.
2. Related Background Arts
When light is incident on an optical fiber, the light is scattered by the vibration of glass structure of the optical fiber, thereby yielding scattering components on the longer wavelength side and the shorter wavelength side relative to the wavelength of the incident light. The scattering component on the longer wavelength side is called a Stokes line, and the scattering component on the shorter wavelength side is called an anti-Stokes line. The power of the Stokes line is stronger than that of the anti-Stokes line. If signal light having the same wavelength as the Stokes line is incident on an optical fiber at the same time as Stokes line occurs, stimulated Raman scattering is generated. The signal light is thereby Raman-amplified while propagating through the optical fiber. That is, the light performs as pump light for Raman amplification. This phenomenon is used for obtaining a Raman amplifier of discrete type or distributed constant type.
A rare-earth-doped optical fiber amplifier is suitable for amplifying light having a wavelength that corresponds to the energy level, whereas a Raman amplifier is characterized in that it can Amplify signal light having any wavelength by selecting a suitable wavelength of pump light for Raman amplification.
In the case of an optical fiber made of silica glass as a main component, the transmission loss becomes the smallest around the 1.55 xcexcm wavelength. Also, the strongest stimulated emission occurs at the wavelength that is 14 THz (about 100 nm) away on the longer wavelength side relative to the pump light.
On the other hand, to avoid the waveform degradation of signal light pulses due to four-wave mixing which is one kind of nonlinear optical phenomenon, the wavelength of signal light and the zero dispersion wavelength of an optical fiber must not overlap each other. Moreover, to allow signal light to be received at an acceptable S/N ratio at the receiving end, and also to avoid the waveform degradation of signal light due to four-wave mixing or cross-phase modulation which is one kind of nonlinear optical phenomenon, the wavelength of signal light and the wavelength of pump light for Raman amplification must not overlap each other.
Under such restrictions an optical communication system proposed at present that uses a Raman amplifier employs the 1.55 xcexcm band as the spectrum band for signal light, using an optical fiber having a finite chromatic dispersion of a few ps.nm xe2x88x921.kmxe2x88x921 at the 1.55 xcexcm wavelength (i.e. non zero dispersion shifted optical fiber). It also employs pump light for Raman amplification near the 1.45 xcexcm wavelength. The zero dispersion wavelength of the non zero dispersion shifted optical fiber is near the 1.50 xcexcm wavelength.
An object of the present invention is to provide a Raman amplifier where the spectrum band can be used efficiently and that is applicable to a Wavelength Division Multiplexing (WDM) optical communication system.
In order to achieve this and other objects, a Raman amplifier according to the present invention comprises an optical fiber allowing signal light to be amplified during propagation therethrough and a pump light supplying means for supplying pump light to the optical fiber. The zero dispersion wavelength of the optical fiber is nearly the same as the wavelength of the pump light. Preferably, the wavelength of the pump light is in the wavelength range where the absolute magnitude of the chromatic dispersion of the optical fiber is equal to or less than 0.3 ps.nmxe2x88x921.kmxe2x88x921.
The pump light may include a plurality of wavelength components. The optical fiber may have a plurality of zero dispersion wavelengths and may be supplied with pump light having wavelengths that correspond to their respective zero dispersion wavelengths. Also optical fiber may have the absolute magnitude of dispersion slope of 0.01 ps.nmxe2x88x922.kmxe2x88x921 or more but not exceeding 0.1 ps.nmxe2x88x922.kmxe2x88x921 at the wavelength of the pump light.
The Raman amplifier may be wound in a coil form and may be stored within a station. In this case, preferably the optical fiber has an effective area equal to or less than 20 xcexcm2, transmission loss equal to or less than 1 dB/km at the wavelength of the signal light, polarization mode dispersion equal to or less than 0.2 ps/kmxc2xd at the wavelength of the signal light, and a length equal to or less than 5 km.
The Raman amplifier may be installed as an optical transmission line between stations. In this case, preferably the optical fiber has an effective area equal to or more than 45 xcexcm2, transmission loss of 0.3 dB/km or less at the wavelength of the signal light, polarization mode dispersion equal to or less than 0.2 ps.kmxe2x88x92xc2xd at the wavelength of the signal light, and a length equal to or more than 10 km.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.